The present invention relates to the combustion of carbonaceous fuels. More particularly, the present invention related to the reduction of oxides of nitrogen and carbon loss from the flue emissions produced from the combustion of carbonaceous fuels. Still more particularly, methods and systems involving both the reduction of nitrogen oxides and carbon loss from the flue emission produced from the combustion of carbonaceous fuels, particularly coal, are provided.
By way of background, the combustion of carbonaceous fuels such as coal generates by-products, including various oxides of carbon, carbon dioxide, carbon monoxide, water vapor, sulfur dioxides, and nitrogen oxides. These primary gases, along with unused oxygen and nitrogen from the air used for combustion, constitute the flue gas. Sulfur dioxide and oxides of nitrogen are significant pollutants released into the atmosphere from the combustion of carbonaceous fuels. These two gases are primarily responsible for acid rain and result in environmental damage. Both sulfur dioxide and nitrogen oxides are subject to governmental regulation, including requirements under the Clean Air Act Amendments of 1990. In addition, oxides of nitrogen not only cause acid rain, but also produce ground level ozone by reacting with volatile organic compounds emitted primarily from internal combustion engines involved with transportation. The ozone produced from nitrogen oxides migrates widely and causes problems in regions removed from the site of production. In view of the public concern over ozone damage, various regulations for reducing the emissions of nitrogen oxides have been enacted and new regulations are becoming more stringent.
Various methods are currently used to reduce nitrogen oxides. Present methods to control the emission of nitrogen oxides include burner air staging, furnace air and flue staging (reburning), flue gas recirculation, selective catalytic reduction, and selective non-catalytic reduction. The main principle of the air staging technique is to reduce the level of available oxygen in zones or regions where oxygen is a critical requirement for the formation of nitrogen oxides. The amount of fuel burnt or combusted at the peak temperature is also reduced. Fuel staging, or reburning, is another method of reducing nitrogen oxides by a part of the fuel above the main combustion zone. The hydrocarbon radicals that are released from this fuel reduce nitrogen oxides from the primary combustion zone. However, it should be noted that the main limitation for this method is that the fuel has to be very reactive because of the short residence time that is available for complete combustion after the reburn fuel is introduced. Therefore, natural gas is primarily used as a reburn fuel, although other fuels such as oil, coal, biomass-based products have been used. Selective catalytic reduction uses chemicals such as NH3 to reduce nitrogen oxides over catalysts that are expensive. Ammonia is a hazardous chemical to handle. Another problem in the industry, high un-burnt carbon, is most often encountered with low nitrogen oxide burners. With low nitrogen oxide burners, as the oxygen concentration is reduced in the near burner zone, the combustion process is delayed leading to high un-burnt carbon. This is usually indicated by and called LOI (Loss on Ignition).
With current reburning or fuel staging methodology, a part of the fuel is admitted downstream of the main combustion zone. This method requires that the fuel be very reactive and with less nitrogen since the time available for complete combustion is short. The problem with using coal as a reburn fuel is that the volatiles produced from combustion can be used to reduce nitrogen oxides, but the char increases the unburnt carbon loss. Accordingly, coal is not a preferred choice.
U.S. Pat. No. 5,967,061 is directed to a method and system for reducing nitrogen oxide and sulfur oxide emissions from carbonaceous flue gases. U.S. Pat. No. 5,045,180 is directed to a process for catalytic multi-stage hydrogenation of coal. U.S. Pat. No. 5,178,101 is directed to a method for reducing oxides of nitrogen generated in a coal-fired fluidized bed boiler. U.S. Pat. No. 5,291,841 is directed to a process for combustion of coal to maximize combustion efficiency while minimizing emissions of sulfur and nitrogen oxides.
In light of current technology, there still remains a long-felt need for methods and systems that reduce nitrogen oxides and carbon loss from carbonaceous fuel combustion flue emissions, especially when coal is the fuel. Methods and systems that are easily implemented with existing carbonaceous fuel combustion systems are particularly desirable.
Accordingly, it is an object of the present invention to provide a method for the reduction of nitrogen oxides from carbonaceous fuel combustion flue emissions.
It is another object of the present invention to provide a method for the reduction of carbon loss from carbonaceous fuel combustion flue emissions.
Yet another object of the present invention is to provide a system for reducing oxides of nitrogen and carbon loss from carbonaceous fuel combustion flue emissions.
Still another object of the present invention is to provide a method and system for the reduction of nitrogen oxides and carbon loss from coal combustion flue emissions.
These and other objects and advantages of the present invention and equivalents thereof, are achieved by the methods and systems of the present invention described herein and manifest in the appended claims.
In accordance with the objects outlined above and other objects, the present invention provides methods and systems for reducing nitrogen oxides and carbon loss from carbonaceous fuel combustion flue emissions. The method of the invention comprises heating a first portion of carbonaceous fuel to a first temperature in a first chamber (external to the main burner system or within the main burner system) sufficient to thermally decompose the carbonaceous fuel to produce a first gaseous stream and a char fraction; adding the char fraction directly to the main burner, or to a second portion of the carbonaceous fuel, and combusting at a second temperature in a second furnace chamber (main burner) to produce a second gaseous stream (combustion products); and adding the first gaseous stream downstream to the second gaseous stream. The thermal decomposition, or low temperature pyrolysis, is at a temperature from about 600xc2x0 C. to about 850xc2x0 C., preferably about 600xc2x0 C. to about 700xc2x0 C. The combustion of the second portion of the carbonaceous fuel is preferably at a temperature from about 1300xc2x0 C. to about 1700xc2x0 C. The preferred carbonaceous fuel is coal. The first portion of the carbonaceous fuel, preferably coal, is in an amount from about 15 wt % to about 50 wt % and the second portion of the carbonaceous fuel is preferably in an amount from about 50 wt % to about 85 wt % of the total weight of the fuel. Preferably, the char fraction from the first chamber and the second portion of the coal is pulverized prior to combusting at the second temperature. In one embodiment, the combined first gaseous stream from thermal decomposition or low temperature pyrolysis (pyrolysis products) are combined with the second gaseous stream (main combustion products) in the first furnace chamber. Carbonaceous fuel is selected form the group of coal, biomass, petroleum coke, bitumen, fuel oil, non-aqueous mixtures thereof, and aqueous mixtures thereof.
The present invention also provides a system for reduction of nitrogen oxides and carbon loss from carbonaceous fuel combustion flue emissions. The system comprises a means for thermally decomposing a first portion of the carbonaceous fuel at a first temperature; a means for combusting a second portion of the carbonaceous fuel at a second temperature (in the main burner); a means for adding the first gaseous stream to the second gaseous stream downstream from the second gaseous stream; and a means for adding the char fraction to the second portion of the carbonaceous fuel prior to combusting the second portion of the carbonaceous fuel. The preferred carbonaceous fuel of the system is coal. The system further comprises a means for recycling combustion flue emissions located downstream from the means for adding the first gaseous stream to the second gaseous stream to the means for thermally decomposing the first portion of the carbonaceous fuel. Also, the system further preferably comprises a means for pulverizing the char fraction and the second portion of the carbonaceous fuel prior to combusting the char fraction and the second carbonaceous fuel portion. The thermal decomposition means, or low temperature pyrolysis means, is preferably at a temperature from about 600xc2x0 C. to about 850xc2x0 C., preferably about 600xc2x0 C. to about 700xc2x0 C. The means for combusting the second portion of the carbonaceous fuel is preferably at a temperature from about 1300xc2x0 C. to about 1700xc2x0 C. The first portion of the carbonaceous fuel of the system is preferably in an amount from about 15 wt % to about 50 wt %, and the second portion of the carbonaceous fuel of the system is preferably in an amount from about 50 wt % to about 85 wt % of the total weight of the fuel.